In the electronics industry, panel fasteners are often assembled to printed circuit boards by a robotic pick-and-place machine, which inserts the fastener into a circuit board receiving hole. This step is most usually done in conjunction with a surface mount soldering process. Even with the accuracy afforded by these machines, assembly errors can occur because the components of the panel fastener are only loosely joined.
Panel fasteners typically comprise a retractable and extendable screw that sits within a ferrule. The screw is held in the retracted position by a light coil spring operatively positioned around the screw shank between the screw head and the top of the ferrule. In the prior art, a large sidewall gap exists between the outer surface of the screw and the inner wall of the ferrule, which accommodates some misalignment between the circuit board and the structure to which it is affixed. The sidewall gap presents a problem however because it causes instability in handling the part by the pick-and-place machine. During assembly, the pick-and-place machine holds the fastener by the head of the screw and rapidly accelerates the fastener during assembly, which causes relative movement between the screw and the ferrule. This rapid movement during assembly can cause the ferrule to be misaligned with the circuit board receiving hole.
The aforementioned assembly problem, along with one proposed solution, is disclosed in U.S. Pat. No. 8,113,755, which discloses the use of a nut or an o-ring attached to the screw on the underside of the ferrule to hold the screw in a downwardly-compressed configuration such as shown in FIG. 1. This configuration eliminates movement between the screw and the ferrule. After the fastener is robotically assembled to the circuit board, the hold-down nut or o-ring is removed to allow the fastener to return to its normal retracted free state. This solution, however, has drawbacks including the additional cost of the nut or o-ring and the need to remove and dispose of those parts after circuit board assembly. Therefore, it would be desirable to provide a panel fastener and panel fastener assembling process that is less complicated and less expensive than the prior art, and can be used successfully with high-speed pick-and-place robotic assembly machines and methods.